The subject invention is directed toward the art of rock drilling bits and, more particularly, to a bit of the type having hard material cutting inserts press-fitted into a steel body, and a method of producing such a bit.
The invention is particularly suited for use in producing rock drilling bits of the type generally referred to as percussion drilling bits and will be described with reference thereto; however, as will become apparent, the invention could equally well be used to produce roller cone bits, polycrystalline diamond compact (PCD) bits, and similar bits of the type wherein the cutting is performed by hard material inserts carried in a steel body.
The typical percussion drill bit comprises a steel bit body having a generally cylindrical mounting shank carrying an axially aligned cylindrical head defining a cutting face. A multiplicity of cylindrical, hard material cutting inserts, generally formed of sintered tungsten carbide, are press-fitted in precision drilled openings in the cutting face. The exposed ends of the cutting inserts perform the actual cutting by abrading or crushing the rock into dust and small particles. The dust and particles are flushed from the drill hole by compressed air or other pressurized fluid supplied through the drill face.
Typically, the life of such bits is dependent on the life of the hard material inserts. However, in certain rock formations, such as soft, fractured formations, the bit body itself is subjected to significant erosive wear. Under those circumstances, failure of the bit often occurs prematurely because of the wearing away of the bit body metal in the area surrounding the inserts. This results in loss of support for the inserts and loss of inserts, especially in the outer or "gage" row of inserts.
In an effort to overcome the noted problem, various attempts have been made to increase the hardness of the steel bit body by using a higher carbon content steel and heat treating for high hardness. Such attempts have typically been unsuccessful because of the difficulty of drilling the precision insert openings in a hardened steel body. Likewise drilling the openings before hardening has not been successful because the subsequent heat treating operations typically produce wide tolerance variations in the opening sizes.
Attempts have also been made to form the bit bodies from a lower carbon content steel and then carburize and case harden the body. Here again, hardening before drilling the insert openings results in significant drilling problems. Carburizing and hardening after drilling not only produces tolerance problems but, also, results in a tendency toward cracking of the hardened case when the carbide inserts are subsequently press-fitted in the openings.